a. Field of the Invention
The present invention relates to a method of treating water containing chromate by bringing the water-to-be-treated into contact with ion-exchange resins thereby to remove said chromate.
B. Description of the Prior Art
Nowadays, it is common to add a chromate-based corrosion inhibitor to the open circulating type cooling water system for the purpose of preventing corrosion of metals employed for said cooling water system. A blowdown containing such a corrosion inhibitor (hereinafter called `cooling-tower blowdown`) usually has a yellow color and contains chromate (hexavalent chromium) of 100 ppm or thereabouts.
The waste water coming out of chromium plating shops employing chromic acid in great quantities also contains noxious chromate.
Polution of rivers and seas attributable to the industrial waste waters has recently become a serious problem, and establishment of some equipment for treatment of waste water is a pressing need at various factories. As regards the cooling-tower blowdown, it also has lately become a subject of strict regulations from the view point of preservation of the quality of public water basins and the environmental quality.
Under such circumstances, separation and removal of chromate from waste water by ion-exchange is common.
This common method is intended to remove chromate by selective adsorption using a weakly basic anion exchange resin in salt form. In this case, the adsorption by a weakly basic anion exchange resin easily occurs when the water-to-be-treated is rather acidic. In order to make the water slightly acidic, an acid or a cation exchange resin is used for pretreatment of the water-to-be-treated.
When chromate ions are adsorbed and removed by the use of a weakly basic anion exchange resin, a satisfactory chromate ion removal effect is realized. However, for the regeneration of the weakly basic anion exchange resin having chromate ions adsorbed thereon, the resin must be treated with alkali agent such as sodium hydroxide, because chromate can not be eluted even when directly treated with an acid solution.
The result of experiments conducted on chromate adsorbed ion-exchange resins containing adsorbed chromate to investigate the regenerativity thereof verifies that, until the chromate elution rate reaches about 85%, chromate can be eluted through the reaction of practical chemical equivalents, but further elution beyond this extent progresses gradually even if excess sodium hydroxide is employed; that is, the elution rate is confined to about 95% even when sodium hydroxide is employed in an amount of 8 times as much as the equivalent quantity of chromate adsorbed on said resin.
It is considered from this result that complete elution of chromate is infeasible, and regeneration of a weakly basic anion exchange resin having chromate adsorbed thereon is actually difficult. The resin after elution of chromate is in OH-form, and when the water-to-be-treated is passed through a bed of this resin, the resin reacts with metal ions (such as Ca.sup.2.sup.+, Mg.sup.2.sup.+, etc.) in the water thereby to bring on deposition of heavy metals on the resin, resulting in clogging and increase in leakage of chromate. To cope with this, it is popular to use a weakly basic anion exchange resin, a part or the entirety of which is converted into salt form by means of an acid, e.g., hydrochloric acid and sulfuric acid or a salt of such acids, e.g., sodium chloride and sodium sulfate after conversion of the said resin into OH-form. In this case, however, chromate adsorbed on the resin is eluted by means of an alkali agent, and regeneration of the resin cannot be performed to perfection, so there occur initial leakage and constant leakage of chromate.